One preferred environment in which the present invention may be utilized is a UPS system. FIG. 1 is a block diagram of such a system. The UPS system depicted in FIG. 1 comprises a rectifier 10 connectable to an AC utility power source, designated AC in FIG. 1. The rectifier 10 provides a DC voltage to an input filter 12 which, in turn, provides unregulated DC voltage to the input of a high frequency resonant converter 14. The resonant converter serves as a current source for an isolation power transformer 16. The power transformer 16 couples the converter 14, and the current from converter 14, to a rectifier 18; the rectifier 18 supplies DC voltage through an output filter 20 to a PWM inverter 22. The current to the transformer is controlled such that the DC voltage output by filter 20 is regulated to the desired voltage. The PWM inverter 22 furnishes an AC voltage to a load (not shown) connected to the UPS system through a low pass filter 24. The UPS system also includes: appropriate control blocks 26, 28 for control of the PWM inverter 22 and the resonant converter 14; sensing devices 30, 32 for sensing the operating current I.sub.R of the converter 14 and the voltage at the output of the output filter 20; a battery 34 connectable to the input of the converter 14 through a switch 36, and a battery charger 38. Typically, power is provided to a constant speed fan (not shown) by a power supply associated with the UPS system. The power supply supplies a fixed voltage that drives the fan at a constant speed regardless of the load on the UPS system.
Known UPS systems have a number of shortcomings. The UPS system depicted in FIG. 1, as well as other known UPS systems, has an inefficient modular topology; i.e., the system includes many separate (non-integrated) functional blocks that do not share parts. The consequences of this modular topology include a high number of parts, high cost, high level of complexity, and low reliability (i.e., low MTFB). In addition, power to run internal cooling fans is difficult to provide, especially where the available AC power varies over wide frequency and voltage ranges and/or where variable fan speed is desired.